Is full functionality there? When I was last doing it, you couldn't use 3G and there were issues with sending/receiving MMS.

I thought the iPhone4 only had GSM bands at 850, 900, 1800, and 1900. T-mobile's GSM bands are 1700 and 2100 IIRC. So that would be why you couldn't get T-Mo 3G on the iP4 even with a SIM unlock. The radio just doesn't run those freqs for that protocol. AFAIK, Edge should be good. This has been the case with iPhones for a while, but I don't know if that changed with the 4S and beyond. For a long time, unlocking AT&T iPhones and selling to T-Mo subscribers has had to come with the caveat that the T-Mo guy can't use 3G.

The iP4 does have HSPA (3.5G) capability at 2100 though. So if you're in a 2100 area, you should be good for UMTS. Otherwise, I think you might be stuck with Edge.